The rotational speed at which a centrifuge rotor is moving may be monitored by the use of a photosensor assembly comprising a phototransmitter, such as a light emitting diode (LED), and a photodetector, such as a phototransistor. The photosensor assembly is mounted with respect to the underside of the rotor element on a mounting ring which is fixed with respect to the rotor. The underside of the rotor carries a speed indicia in the form of a banded speed decal having alternate radially extended contrasting colors thereon. One of the bands of the decal is light reflective while the other band is light absorptive. The passage of the alternately colored bands past the photosensor assembly results in the generation of a pulse train the frequency of which is representative of the rotational speed of the rotor.
The phototransmitter and the photodetector are typically mounted in the same housing and must be carefully positioned with respect to the undersurface of the rotor. This care is required because the photodetector portion of the photosensor assembly is particularly position sensitive. That is to say, slight misadjustments in the optimum spacing between the underside of the rotor element and the photosensor assembly may result in the photodetector losing sensitivity and thereby being unable to accurately respond to the radiation reflected from the speed indicia. As an example of the care required, it is not uncommon to require accuracy on the order of ten thousandths (0.010) of an inch when mounting the photosensor assembly. However, because of tolerance variations present in the various constituent elements of the centrifuge apparatus, meeting this tolerance level requires accuracy in a relatively large number of various dimensional adjustments in order to insure the optimal positioning of the photosensor assembly with respect to the rotor. Of course, such dimensional criticality mandates an extremely costly manufacturing procedure, which is perceived to be disadvantageous.
Accordingly, it is believed to be desirable to provide a mounting arrangement for the photosensor assembly to facilitate the mounting and adjustment of the position of the photosensor assembly with respect to the rotor.